1. Field of the Invention
The present invention relates to a color cathode ray tube, and more particularly to a so-called flat-panel-type color cathode ray tube having a panel in which an equivalent radius of curvature of an outer surface thereof which constitutes a screen is larger than an equivalent radius of curvature of an inner surface thereof.
2. Description of the Related Art
As a picture tube of a television receiver set and a monitor tube of a personal computer or the like, recently, a color cathode ray tube which is referred to as “flat panel type” or “planar panel type” has been popularly adopted. The flat panel type color cathode ray tube includes a vacuum envelope which is constituted of a panel which is provided with a phosphor layer on an inner surface thereof, a neck which houses an electron gun and a funnel which connects the panel and the neck. On the inner surface of the panel, in general, a phosphor layer is formed by applying phosphors of three colors consisting of red (R), green (G) and blue (B) in a mosaic shape or a stripe shape by coating. A color selection electrode (here, referred to as “shadow mask”, hereinafter, the color selection electrode being explained as a shadow mask) is arranged close to the phosphor layer.
The shadow mask is of a self-standing shape-holding type which is formed by a press, wherein the shadow mask has a periphery thereof welded to a mask frame and is supported in a suspended manner on stud pins which are mounted on an inner wall of a skirt portion of the panel in an erected manner by way of suspension springs. Here, a magnetic shield is mounted on an electron gun side of the mask frame. A deflection yoke is exteriorly mounted on a transition region between the neck and the funnel of the vacuum envelope. By deflecting three modulated electron beams which are irradiated from the electron gun horizontally (X direction) and vertically (Y direction), the electron beams are scanned two-dimensionally on the phosphor layer thus reproducing images.
This flat panel type color cathode ray tube is, in view of a manufacturing cost and the easiness of manufacturing, configured such that the outer surface (also referred to as “image forming face”, “screen”, “face” or the like) of the panel has a large radius of curvature (equivalent radius of curvature), that is, the outer surface is made substantially flat, while the inner surface of the panel which constitutes a phosphor layer has a relatively small radius of curvature (equivalent radius of curvature) to an extent that a flat feeling of a display image is not damaged when the display screen is observed from the outer surface of the panel.
For example, with respect to a color cathode ray tube having a diagonal size of the screen of a nominal 29 type, a wall thickness of the panel is set to 12.5 mm at a center portion thereof and 25 mm at a peripheral portion thereof and hence, the difference in wall thickness is large between the center portion and the peripheral portion. Further, as a material which constitutes the panel, that is, as a panel base, a so-called tinted glass is used. Accordingly, the transmissivity of the panel is 51% at the center portion and 28% at the peripheral portion and hence, the difference is large. Accordingly, the brightness of the peripheral portion is approximately 50% of the brightness of the center portion whereby the brightness difference between the center portion and the peripheral portion when the image is displayed is large.
As a method which overcomes such a drawback, as disclosed in JP-A-2001-101984 (hereinafter referred to as “patent document 1”), there has been known a method in which a wavelength selective absorption layer which uses pigment or dye is applied to an outer surface of the panel so as to impart the gradation to the transmissivity of the wavelength. However, when the gradation is imparted using the wavelength selective absorption layer, coloring (saturation of color) of an appearance color (body color: depending on color of phosphor per se) of the screen in a state that the color cathode ray tube is not operated is strong and hence, the color difference between the center portion and the peripheral portion is increased and this is observed as the color irregularities whereby the quality of the color cathode ray tube is lowered. Here, as literatures which disclose other prior art relevant to the surface treatment of the panel, JP-A-2001-210260 (hereinafter referred to as “patent document 2”), JP-A-3-254048 (hereinafter referred to as “patent document 3”), JP-A-2000-258625 (hereinafter referred to as “patent document 4”), JP-A-2001-66420 (hereinafter referred to as “patent document 5”), JP-A-1-320742 (hereinafter referred to as “patent document 6”) and the like can be named.